Djokovic beat Alexandr Dolgopolov 6-1, 6-4 to improve his clay-court record this season to 8-1 and Williams routed 14th-seeded Dominika Cibulkova 6-0, 6-1 for her 21st consecutive win, matching the best streak of her career.

Nadal, however, was far from his best, especially in the first set, in which Gulbis jumped out to a 5-0 lead and even had a point to close out the set at love.

But Nadal, who has reached the final in all seven tournaments he has played since returning to the circuit in February from a seven-month break due to a left knee injury, eventually began to win the longer points while the carefree Gulbis started to commit more errors.

Questions

Still, the performance raises questions about whether Nadal will be able to defend his title at the French Open, the year's second Grand Slam, which starts in 10 days.

While Nadal won last week's Madrid Open, he saw his run of eight consecutive titles in the Monte Carlo Masters end last month with a straight-set loss to Djokovic in the final.

Nadal said earlier this week that he now trains for about 50 minutes a day compared to two hours a year ago.

However, Gulbis had given Nadal trouble before, also taking a set off the Spaniard in four of their previous five meetings, all of which Nadal won.

For the first half of this match, the 6-foot-3 Gulbis dominated with his big serve and forehand while Nadal struggled with a series of forehand errors.

At one point late in the first set, Nadal didn't even move when Gulbis returned his serve within reach.

Gulbis held serve in all 17 games of his first two main-draw matches and Nadal didn't break him until he took a 5-3 lead in the second set.

At times, Nadal was returning so far behind the baseline that Gulbis was able to step into the court and push Nadal to the corners.

The 24-year-old Gulbis has had success in Rome before, upsetting Roger Federer in the second round in 2010 and reaching the semi-finals, where he lost a close three-set match to Nadal.

Gulbis won his third career title in Delray Beach, Florida, in March as a qualifier and his ranking is up to No. 46 this week after finishing last year at No. 135.

It was the third time in 303 clay matches that Nadal lost the opening set 6-1, according to the ATP. The others he also came back to win - in the 2011 Davis Cup against Juan Martin del Potro and in the 2006 French Open against Roger Federer.

Nadal will next face fourth-seeded David Ferrer, who advanced when Philipp Kohlschreiber of Germany withdrew due to dizziness.

Easy ride

The only danger Djokovic faced came while serving at 3-2 in the second set. He saved two break points - one with a big serve down the middle and the other with some crisp shot-making that forced Dolgopolov to net a backhand.

"He's skilled but he made a lot of errors today,'' Djokovic said.

"I did well from the beginning of the match, found my rhythm and took control right away.''

The 23rd-ranked Dolgopolov dropped to 3-22 in his career against top-10 opponents.

The start of the match was delayed for about 30 minutes because of rain.

Djokovic will next face sixth-seeded Tomas Berdych, who defeated Kevin Anderson of South Africa 7-5, 6-2.

Djokovic again showed no signs of the right ankle injury that slowed him during last week's Madrid Open, when the Serb lost in the second round to Grigor Dimitrov.

"I haven't felt any pain or discomfort on court for a week already so that's great news,'' Djokovic said.

Djokovic has reached the Rome final four of the five past years, winning the title in 2008 and `11.

With local hope Sara Errani of Italy playing in the main stadium, Williams was forced to play on an outer court. She didn't make the appearance last too long, winning in 61 minutes.

Williams has twice before won 21 consecutive matches, although both of those streaks came more than a decade ago, in 2002 and 2003.

The 15-time Grand Slam singles winner has won her previous three tournaments.